Abstract

Rotating Instability (RI) is an unsteady aerodynamic phenomenon occurring at off-design conditions in axial compressors, featuring side-by-side peaks below the blade passing frequency (BPF) in pressure spectra. Especially when the mode orders of RI are close to the blade number the interaction effect could generate intense tip clearance noise, which could not be cut-off by the duct. Moreover, RI is regarded as a potential indicator for stall and surge. According to previous studies the mechanism of RI could be classified as the unsteady vortex system theory and the shear layer instability theory. This paper presents an experiment in a low-pressure single-rotor compressor, comparing the RI characteristics with two different tip clearances at different operating conditions. A total of 28 Kulite transducers were circumferentially mounted on the casing wall to measure the pressure fluctuation in the two configurations. Utilizing the compressive sensing (CS) method based on the double-uniform sampling point (DUSP) technique the mode orders of RI could be accurately determined. Through throttling the RI phenomenon was observed in different range of flow rates under both tip clearance configurations. The evolution pattern of RI is profoundly affected by the tip clearance size, i.e., the RI phenomenon only presents a two-stage “strengthen-weaken” pattern with the nominal gap, while shows a three-stage “strengthen-weaken-strengthen” pattern with the larger gap. Moreover, the mode characteristics of RI in the frequency domain are analyzed. Based on the experimental results several discussions on the mechanism of RI are proposed. These findings do not conform with the unsteady vortex system theory, but provides evidence for the shear layer instability theory.

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